Advantaged at environmental protection, high luminance, low power consumption, long service life, low working voltage and easy integration, light-emitting diode (LED) is the fourth-generation new light source following incandescent lamp, fluorescent lamp and high intensity discharge lamp.
The LED cannot transfer electric energy into light by 100%, and the rest energies will transfer into heat. Thermal aggregation increases LED temperature and sharply reduces luminous flux, thus resulting in color point shifting and great shortening of service life. For this reason, heat dissipation remains a key problem of LED, in particular in large power LED, from which, large heat is generated due to large power.
LED heat is mainly dissipated through heat conduction, and its heat dissipation capacity is determined by thermal resistance. Therefore, reduction of thermal resistance is a main method to solve heat dissipation problem. Sapphire substrate and die bonding paste are two main factors that impede reduction of LED thermal resistance. Low thermal conductivity of the sapphire substrate can be solved by mainstreaming vertical chip (Si substrate or alloy substrate) or flip chip yet their applications are restricted by complex process and high cost. For die bonding paste like M2 glue in common use, its 0.2 W/mK thermal conductivity significantly reduces heat dissipation. And silver adhesive, as an epoxy matrix, has poor reliability despite its high thermal conductivity.